The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Some IEEE 802.11 standards use a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism to access wireless medium (e.g., air, hereinafter medium). A client station (STA) typically listens to the medium (i.e., senses the medium) before transmitting a signal on the medium. There are two methods for sensing the medium. A first method includes physical carrier sensing, which detects presence or absence of RF energy in the medium. A second method includes virtual carrier sensing, which detects presence or absence of an 802.11 signal in the medium.
Clear channel assessment (CCA) is a logical function performed in a physical layer (PHY) of a STA or an access point (AP) that determines a current state of use of the medium. For example, in wireless devices (AP/STA) that use orthogonal frequency division multiplexing (OFDM), a CCA level (threshold) for a 20 MHz channel is typically −82 dB, and the CCA level (threshold) for a 40 MHz channel is typically −79 dB. If an energy level detected in a channel is less than the CCA level for the channel, the channel is considered idle, and a transmission in the channel can be attempted. If the energy level detected in the channel is greater than the CCA level for the channel, the channel is considered busy, the wireless device may backoff, and a transmission may not be attempted to avoid collisions.